Download Lecture 11 Aerosol Generation and Measurements

AOSC 634
Aerosol Generation and Measurement
Material from James Smith and Steven Massie
[email protected], [email protected]
NCAR is sponsored by the National Science Foundation
Outline
• Aerosol generation techniques
– electrospray
– atomization
– vibrating orifice aerosol generator
• Aerosol physical properties (number, size)
– condensation particle counter
– differential mobility analyzer
– optical particle counter
• Aerosol optical properties
• Aerosol chemical composition
–
–
–
–
Nanometer-sized particle composition
Aerosol mass spectrometer
Tandem differential mobility analyzer
TDCIMS (thermal desorption chemical ionization mass spectrometer)
Aerosol generation
(smallest to largest particle sizes)
Taylor cone
Wikipedia
Expose a small volume of electrically conductive liquid to an electric field in
a capillary tube of ~ mm diameter.
When a threshold voltage is exceeded, the slightly rounded tip emits a jet of liquid.
The droplets disintegrate and spread apart due to electrostatic repulsion.
These devices are used in low power thrusters on spacecraft.
Electrospray particle generator: dp = ~ 5 – 50 nm
neutralizer used to stop
fission process
Neutralizer
Natural aerosols frequently are charged
To transport aerosol particles, it is important to neutralize them
Use e.g. a TSI instrument to do this with a Kr-85 or Po-210 source
Radioactive source ionizes surrounding air into positive and
negative ions. These ions interact with the aerosol particles
Particles discharge by interacting with the ions
aerosol atomizer: ~ 20 nm to 0.5 mm
• the particle size changes
with respect to air
velocity, viscosity and
surface tension
• need to include a dryer
downstream
• at small sizes
contamination may be
an issue
Hinds
Vibrating orifice aerosol generator (VOAG): ~1 – 200 mm
1/ 3
 6QL 

d p  
 f 
piezoelectric actuator
diameter can be changed by changing flow
rate or frequency to piezo.
Q = flow rate, f = frequency
Hinds
Aerosol physical properties
(mostly size and number)
Aerodynamic Diameter
Consider an aerosol particle.
Its Aerodynamic Diameter is the diameter of a water droplet
that falls at the same speed as the aerosol particle
Water
1 gm / cm3
Hinds
Other ways of measuring size distribution or making sizeclassifications
• inertial-based methods – see tutorial:
http://aerosol.ees.ufl.edu/instrumentation/section01.html
cascade impactors
cyclone separators
Inertia based instruments
An Impactor separates the particles into two size ranges,
larger or smaller than a cutoff size
Cascade impactor: have multiple impaction stages in series
(largest cutoff size is 1st stage, etc). Decrease the nozzle size each
stage. Can get access to each impaction plate and then weigh
the particles.
Virtual impactor: replace the impaction plate with a collection
probe. Particles with sufficient inertia go into the collection
probe.
Time – of – flight: have a nozzle emit particles, use two lasers
at e.g. 100 mm apart used to time the particles travel. Particle’s
Aerodynamic diameter is based upon it’s travel time between the
two beams.
Optical Particle Counter (OPC): ~ 100 nm to 5 mm
size limits defined by Mie scattering, which are used to interpret integrated scattered
intensity.
Advantages:
• Can detect very small particles
• Non-intrusive
• Instantaneous and continuous
information
Disadvantages:
• too sensitive to small changes in
• refractive index
• scattering angle
• particle size
• particle shape
Condensation Particle Counter
Saturate an aerosol with water or alcohol vapor
Cool by adiabatic expansion or flow through a cold tube
Nuclei will grow to ~ 10 mm
Every nuclei grows to a droplet
Measure the number of droplets with an e.g. single particle
optical counter
Condensation Particle Counter (CPC): ~1.5 nm to 0.5 mm
Condensation Particle Counters (CPCs) detects
particles by exposing them to a region that is
supersaturated with vapor (usually butanol),
thus allowing particles to grow to a size that
can be optically detected.
Counting efficiency curve: TSI model 3010
Response time: TSI model 3010
Signal to Particle Diameter
Hinds
DMA - Differential Mobility Analyzer
A charged particle will be pushed in the direction of VTE by the
electric field E between the two plates.
Hinds
DMA - Differential Mobility Analyzer
Stokes Drag on a particle
Fd = 3   V d / Cf
 = viscosity of air
V = transverse velocity (going from plate to plate)
d = diameter of the particle
Cf = 1 + (mean free path of particle) / d
(correction factor)
Electric force on a particle with charge Q in electric field E is
QE
Equate the two forces , solve for
V=QEB
V = Q E Cf / 3   d
where B is called the Mobility
Hinds
Differential Mobility Analyzer (shown below, a “Nano DMA”)
Efficiently size-selects charged
particles for collection and
analysis.
inlet
sheath air
HV
outlet
TSI, Inc.
Chen et al., 1998
Unipolar charger
An efficient ambient
nanoparticle charger
~x10 more efficient than
bipolar chargers for
sub-20nm particles.
210Po
source
rings, coupled by
resistors
Voltages turned off for
particles >20nm due to
multiple charging.
Chen & Pui, 1999;
Smith, et al., AS&T, 2004
DMA + CPC = Scanning Mobility Particle Sizer (SMPS) or Differential
Mobility Particle Sizer (DMPS)
DMPS:
• A pre-impactor removes all particles larger than the upper diameter of the size range to be
measured
• The particles are brought in the the bipolar charge equilibrium in the bipolar diffusion charger.
• A computer program sets stepwise the voltage for each selected mobility bin.
• After a certain waiting time, the CPC measures the number concentration for each mobility bin.
• The result is a mobility distribution.
• The number size distribution must be calculated from the mobility distribution by a computer
inversion routine.
Aerosol Optical Properties
Scattering Geometry
=scattering angle
Note polarization:
|| Parellel to scattering
plane
 Perpendicular to
scattering plane
Bohren and Huffman
Phase function
P( ’, ’’ ) = Phase function
1 = (1/4 )  P ( ’, ’’ ) d 
Given the direction ’ of an incident beam, and direction
’’ of the scattered direction, the scattering angle  = ’ - ’’
 < 90 for forward scattering
 > 90 for backward scattering
The phase function tells you the 3 dimensional angular pattern
of the scattered light
See Thomas and Stamnes, Radiative Transfer in the Atmosphere
and Ocean, Cambridge University Press, 1999.
Phase Functions
Scattering Angle
is ’ - ’’
Ice particles
have sharp
forward
scattering
peak
Thomas and Stamnes, Fig 6.3
Particle Scattering Patterns
X=  D / 
D = particle diameter
 = wavelength of light
Bohren and Huffman
Mie scattering
Measurement of optical properties: Extinction
Beer’s Law
I
 exp(  e L)
I0
Extinction Coefficient
Nd p2Qe
 e  NAp Qe 
4
(monodisperse aerosols)
Extinction Efficiency
Qe 
radiant power scattered and absorbed by a particle
radiant power geometrica lly incident on the particle
L = path length,
N = number of particles per volume
Extinction-based aerosol instruments
transmissometer
(used at airports)
pulsed laser cavityringdown spectrometer
stack opacity monitor
Nephelometer: Measuring light scattering
The nephelometer is an instrument that measures aerosol light scattering. It detects
scattering properties by measuring light scattered by the aerosol and subtracting light
scattered by the gas, the walls of the instrument and the background noise in the detector.
Aerosol chemical composition
Tandem Differential Mobility Analyzer (TDMA)
if volatile ...
Dp
Dp
Dp
Heater
Particle
Counter
DMA1
DMA2
Humidifier
if hygroscopic ...
Dp
Dp
Dp
Volatility (~100 °C)
Hygroscopicity
Sulfuric acid
Volatile
Very hygroscopic
Sulfates
(Totally or partially
neutralized by ammonia)
Non-volatile
Very hygroscopic
Volatile
Not or only slightly
hygroscopic
Organic carbons
Mass Spectrometer
Have a charged molecule of charge Q
Impose E and B fields. The molecule will spiral in the fields.
F = Q (E + (v x B ))
(Lorentz force)
The curvature of the path of the molecule is given by F = m A
with A the acceleration (e.g A = v2 / R with radius of curvature R)
(m/Q) A = E + ( v x B )
Express Q = z e
Mass spec data has m / z on the x axis of a graph
Mass Spectrometer
Old style mass spec
Have B direction
perpendicular
to the page
Use “Right hand
Rule” to see the
direction of
vxB
Radius R of path
of particle of
larger mass differs
from that of the
lighter particles
Wikipedia
Quadrupole Mass Analyzer
Wikipedia
Radio frequency voltages are applied between one pair of rods and the other
Only ions of a certain mass-to-charge ratio will pass through the quadrupole (others
will collide with the rods)
Aerosol Mass Spectrometer (AMS)
• For an excellent review of this and other instruments that
measure aerosol composition using mass spectrometry see:
– http://cires.colorado.edu/~jjose/Papers/201009_IAC_Aerosol_MS_Tutorial.pdf
Field observations
Lab
Lab
Canagaratna et al.,
Mass Spec Rev, v26,
P185-222,2007.
AMS mass spectrum from ambient aerosol
•
•
•
Since the AMS uses electron impact ionization and high temperature, species are
modified as they are desorbed and ionized.
Luckily, marker species and co-varying peaks can be found that uniquely identify
compound classes.
A high-resolution Time-Of-Flight Mass Spectrometer (TOFMS) has been developed
for use with the AMS, thus allowing for elemental analyses such as C:O. In the
TOFMS, an E field accelerates ions of different mass to the same kinetic energy ½
m v2. Larger mass ions travel at slower v than lighter ions. For each ion, measure
the travel time between two laser beams, get v, and then m.
Thermal Desorption Chemical Ionization Mass
Spectrometer (TDCIMS)
Use electrostatic precipitator to collect particles
Use evaporation-ionization chamber to ionize particles
A collision-induced dissociation (CID) chamber is used
to strip clusters down to their ion cores
Use triple quadrupole mass spectrometer to sort the
particles
Electrostatic precipitator
Place a wire in a tube
Have E field between wire and tube
In the TDCIMS, charged particles go to the wire
Chemical Ionization
H3O+ + NH3 -> NH4+ + H2O
CID - Collision-induced dissociation chamber
Accelerate ions and let them collide with neutral
e.g. Argon gas. The ions will break apart.
Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS)
an instrument for characterizing the chemical composition of
ambient particles from 8 to 50 nm in diameter
Voisin et al., AS&T, 2003; Smith, et al., AS&T, 2004
TDCIMS electrostatic precipitator
no voltage applied to filament
Flows of clean N2 keep ambient
air away from ion source and
filament.
de-clustering cell
ion source
Concentration of particles
exiting precipitator noted for
estimating collected fraction.
collection filament
size-selected
nanoparticles from
Nano-DMA
mass
spec.
TDCIMS electrostatic precipitator
4000 V applied to filament
Charged particles are attracted
to the filament by the electric
field.
Collection is done at RT and
atm, for ~5 – 15 min in order to
collect ~10-100 pg sample.
Concentration of particles
exiting precipitator noted for
estimating collected fraction.
TDCIMS electrostatic precipitator
Charged particles are attracted
to the filament by the electric
field.
Collection is done at RT and
atm, for ~5 – 15 min in order to
collect ~10-100 pg sample.
Concentration of particles
exiting precipitator noted for
estimating collected fraction.
collection complete
filament moved into ion source
TDCIMS ion source
•
•
Pt wire ramped from room
temperature to ~550 °C to
desorb sample
Close-up of ion source during
sample desorption
pinhole to vacuum
chamber
Neutral compounds are ionized
using chemical ionization,
e.g.: (H2O )nH3O+ + NH3
(H2O )mNH4+ + (H2O)n-m

•
Reagent ions are created by a
particles emitted from the
source, generating mostly H3O+,
O2- and NO-, …
•
Ionized analyte injected into a
triple quadrupole mass
spectrometer for analysis
241Am
foil
de-clustering
cell
Pt filament
Temperature programmed TDCIMS: Dicarboxylic
Soft ionization
of dicarboxylic
Acid
Results C = acids
4-8
Suberic acid C8
Dicarboxylic acids like
to fragment, typically into
formic acid (HCOOH), which has a
mass of 46 amu units.
Current
Parent
M-46 fragment
Pimelic acid C7
~100 Hz per pg collected aerosol
Normalzed Counts
Adipic acid C6
Glutaric acid C5
Succinic acid C4
Filament current
550 °C
0
0
Values are integrated areas of
curves on the right
300
Time (sec)
Smith and Rathbone, Int. J. Mass Spectrom., 2008
References
William Hinds, Aerosol Technology – Properties, Behavior, and
Measurement of Airborne Particles, John Wiley, 1999
Air Sampling Instruments for Evaluation of Atmospheric
Contaminants (9th ed), by American Conference of Governmental
Industrial Hygenists Staff, 2001
Baron and Willeke, Aerosol Measurement, 2005